Inspired by the classic tarte flambee from Alsace and by that northern French region's love for pairing ham with cheese, here's an easy appetizer with wide appeal. It's also a great way to use up leftover ham.
Preheat oven to 425 degrees, with a baking sheet on rack in lower third. Heat oil in a skillet over medium. Add onion and a pinch of kosher salt. Cook, stirring often, until onion is caramelized, 18 minutes. Spread ricotta onto flatbread. Sprinkle evenly with fontina. Top with onion and ham. Bake on heated sheet until cheese has melted, 10 minutes; switch oven to broil and cook until cheese is bubbly, 2 to 3 minutes more. Let cool slightly; garnish with flat-leaf parsley, slice, and serve.